#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
	int n;
	double x, sum = 0;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> x;
		sum += 1 / x;
	}
	cout << fixed << setprecision(2) << n / sum;
	return 0;
}
